The above mixture of oil and litharge, gently and carefully boiled in an open vessel till it thickens, becomes strong drying oil for dark colors. Boiled oil is sometimes set on fire purposely, in making printer's varnish and printing ink, and also for painting and the preparation of japanner's gold size. As dark and transparent colors are in general comparative ill driers, japanner's gold size is sometimes employed as a powerful means of drying them. This material may be prepared in the following manner: Asphaltum, litharge or red lead, burnt umber or manganese, finely powdered, of each 1 ounce; stir them into a pint of linseed oil and simmer the mixture over a gentle fire, or on a sand bath, till solution has taken place, scum ceases to rise, and the fluid thickens on cooling, carefully guarding it from taking fire. If the oil employed be at all acid or rancid, a small portion of powdered chalk, or magnesia, may be usefully added, and will assist the rising of the scum and the clearing of the oil by its subsidence; and, if it be kept at rest in a warm place, it will clear itself, or it may be strained through a cloth and diluted with turpentine for use.
There is often a difficulty in obtaining the oils bright; after boiling or heating them with the lead solutions, the best way, on a small scale, is either to filter them through coarse woollen filtering paper, or to expose the bottle for some time to the action of the sun, or to place it in a warm situation; on a large scale, the fine oils are often filtered through Canton flannel bags. The litharge and sulphate of lead used in the above processes may be again rendered available for the same purpose by washing them in hot water to remove the adhering mucilage.